The present invention relates to density logging by gamma ray irradiation of an earth formation and detection of back-scattered gamma ray radiation. More particularly, the invention concerns methods and apparatus for determining the densities of earth formations surrounding a borehole, with an improved compensation for the effect of mudcake on the borehole wall.
It is known to measure the densities of geological formations surrounding a borehole by irradiating these formations with gamma rays from a collimated source contained within a logging sonde and by detecting back-scattered gamma rays; that is, by detecting gamma rays that have been scattered back to the borehole. As the gamma rays from the source interact principally with free or loosely bound electrons, the number of the gamma rays returning to the sonde depends upon the formation density which can thus be determined.
One of the major problems encountered in density logging arises from the presence of mudcake on the borehole wall. As the mudcake is traversed twice by the detected gamma rays, the formation density derived from the count rate of these gamma rays is strongly affected by the mudcake density which is generally different from the formation density.
To obtain correct density measurements, it has previously been proposed to use two detectors spaced at different distances from the source. The near, or short-spaced, detector receives gamma rays which have diffused mainly in the materials near the borehole wall, and therefore in the mudcake. The far, or longspaced, detector receives gamma rays which have diffused principally in the formation. Moreover, in order to eliminate the influence of the average atomic number of the elements in which the radiation diffuses, the count rate of the back-scattered gamma rays detected by the detectors is measured in a restricted energy range chosen such that Compton scattering is the only consequential form of interaction with the adjacent materials. A value of uncompensated, or apparent, formation density is derived from the long-spaced count rate and a density correction is computed using both the long- and short-spaced count rates. This correction is then applied to the uncompensated value to obtain the corrected formation density. One such arrangement is described in U.S. Pat. No. 3,321,625, issued on May 23, 1967 to John Wahl.
A substantial improvement of the above method consists of both collimating the short-spaced detector and measuring its count rate in a restricted energy range covering relatively little degraded singly-scattered gamma rays, so as to increase the sensitivity to gamma rays interacting with materials contained in a thin layer immediately adjacent to the borehole wall, which may or may not include mudcake. An excellent compensation can thus be obtained for the effect of this layer, whose thickness is, typically, about 30mm. However, it is quite easy to understand that, if the above technique provides an increased sensitivity to the layer of materials immediately adjacent to the borehole wall, it also considerably reduces the compensation for the effect of the materials beyond this layer. As the mudcake thickness rarely exceeds 30mm, this limitation has consequences mainly in the case of altered shales. It frequently happens that water is absorbed by the shales over considerable thicknesses, which most often exceed 30mm. A layer of reduced density, amounting to a very thick mudcake, is thus created. As the effect of this layer is only partially corrected, the shale density measured in too low, which constitutes a serious disadvantage when one considers the importance of shales in the techniques for interpreting measurements made in boreholes.
One way of overcoming this drawback consists of reducing the lower limit of the energy range of the short-spaced count rate in order to cover back-scattered gamma rays of lower energy, which have therefore penetrated more deeply into the materials adjacent to the borehole wall. Such a solution is not satisfactory, for while increasing the depth of compensation, it also decreases very substantially the quality of the compensation for the effect of materials very near the wall. Since in most cases mudcakes have a small thickness, such a compromise is not acceptable.
It is therefore an object of the present invention to provide density logging methods and apparatus with a satisfactory compensation both for the effect of materials immediately adjacent to the borehole and for the effect of materials near the borehole but not immediately adjacent to its wall.